The regional impact of urban emissions on climate over central Europe: present and future emission perspectives

Abstract. The regional climate model RegCM4.2 was coupled to the chemistry transport model CAMx, including two-way interactions, to evaluate the regional impact of urban emission from central European cities on climate for present-day (2001–2010) and future (2046–2055) periods, and for the future one only emission changes are considered. Short-lived non-CO₂ emissions are considered and, for the future impact, only the emission changes are accounted for (the climate is kept “fixed”). The urban impact on climate is calculated with the annihilation approach in which two experiments are performed: one with all emissions included and one without urban emissions. The radiative impacts of non-CO₂ primary and secondary formed pollutants are considered, namely ozone (O₃), sulfates (PSO₄), nitrates (PNO₃), primary organic aerosol and primary elementary carbon (POA and PEC).

The validation of the modelling system is limited to key climate parameters, near-surface temperature and precipitation. It shows that the model, in general, underestimates temperature and overestimates precipitation. We attribute this behaviour to an excess of cloudiness/water vapour present in the model atmosphere as a consequence of overpredicted evaporation from the surface.

The impact on climate is characterised by statistically significant cooling of up to −0.02 and −0.04 K in winter (DJF) and summer (JJA), mainly over cities. We found that the main contributors to the cooling are the direct and indirect effects of the aerosols, while the ozone titration, calculated especially for DJF, plays rather a minor role. In accordance with the vertical extent of the urban-emission-induced aerosol perturbation, cooling dominates the first few model layers up to about 150 m in DJF and 1000 m in JJA. We found a clear diurnal cycle of the radiative impacts with maximum cooling just after noon (JJA) or later in afternoon (DJF). Furthermore, statistically significant decreases of surface radiation are modelled in accordance with the temperature decrease. The impact on the boundary layer height is small but statistically significant and decreases by 1 and 6 m in DJF and JJA respectively. We did not find any statistically significant impact on precipitation and wind speed. Regarding future emissions, the impacts are, in general, smaller as a consequence of smaller emissions, resulting in smaller urban-induced chemical perturbations.

In overall, the study suggest that the non-CO₂ emissions play rather a minor role in modulating regional climate over central Europe. Much more important is the direct climate impact of urban surfaces via the urban canopy meteorological effects as we showed earlier.